smtp0.web4u.cz has no MX records.
Question and response
Question
dig smtp0.web4u.cz. MX
Answer
The response has no answer records.
Authority
web4u.cz. 60 SOA ns.web4u.cz. admin.web4u.cz. 2026021711 14400 3600 1209600 14400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0